1. Field of the Invention
This invention relates to a process for the preparation of an indigo compound by reacting an indol compound which is unsubstituted at the 2- and 3-positions with an organic hydroperoxide.
2. Description of the Prior Art
Indigo compounds are important compounds that are useful as dyes. The presently employed industrial processes for the preparation of indigo comprise forming an N-phenylglycine salt from aniline and chloroacetic acid, or aniline, cyanic acid and formaldehyde, converting this salt into an indoxyl compound by alkali fusion at elevated temperature, and then oxidizing this compound with air. However, these processes are not only complicated ones having many steps, they also require the use of large amounts of potassium hydroxide and sodium hydroxide. Moreover the recovery and reuse of used potassium hydroxide and sodium hydroxide has the disadvantage of consuming much energy and requiring special equipment. Therefore, conversion to a simpler process has been desired.
Meanwhile, it has been reported in the field of synthetic organic chemistry that indigo was formed by oxidative dimerization of indole. Specifically, Obata et al. have reported that, when peracetic acid, which is a percarboxylic acid, was produced from hydrogen peroxide and acetic acid in the reaction system and reacted with indole, a trimer of indole, or 2,2-diindyl-.PSI.-indoxyl, was obtained and, in addition, a small amount of indigo was formed as a by-product (Bull. Agr. Chem. Soc. Japan, Vol. 20, pp. 80-83, 1956). Moreover, B. Witkop et al. have reported that, when perbenzoic acid, which is a percarboxylic acid, was reacted with indole in a chloroform solvent by allowing the reaction mixture to stand in a refrigerator overnight, a very small amount of indigo was formed together with a variety of other products (Justus Liebigs Annalen der Chemie, Vol. 558, pp. 91-98, 1947). Furthermore, A. K. Sheinkman et al. have reported that, when hydrogen peroxide, which is an inorganic peroxide, was reacted with indole in methanol, the trimer 2,2-diindyl-.PSI.-indoxyl was obtained in high yield, as was the case with the reaction using peracetic acid, and the formation of indigo was only detected by chromatography (Khim. Geterotsikl. Soedin., Vol. 11, pp. 1490-1496, 1978).
However, the object of these reports was to study the reactivity of indole, and the indigo that is the desired product in the present invention was nothing but a by-product formed in very small amounts. Accordingly, these are not satisfactory processes for the preparation of indigo compounds.